1. Field of the Invention
The present invention relates to a pattern inspection method, a pattern inspection apparatus and a semiconductor device manufacturing method, and is intended for a pattern inspection using, for example, a charged particle beam.
2. Related Background Art
In an inspection of a circuit pattern of a semiconductor device, a so-called Die-to-Database inspection using a scanning electron microscope has recently been in wide use. The Die-to-Database inspection comprises scanning a wafer with an electron beam and then detecting a secondary electron, a reflection electron and a back scattering electron generated from the surface of the wafer to acquire an SEM image of a circuit pattern, measuring the dimensions of the pattern in the SEM image referring to computer aided design (CAD) data, and comparing the measurement with the CAD data to inspect its quality.
The inspection with the electron beam requires a long time, and there is therefore a demand for a faster inspection in particular. One method available is to increase a scan region of the electron beam to collectively acquire an SEM image of a wide range. Another method is to increase a probe current to scan with the electron beam at a high velocity, and obtain a satisfactory S/N ratio even with a small accumulation number.
However, when the scan region is wide-ranging, the diameter of the beam changes between the center and end of the scan region due to the deflection and aberration of the electron beam, so that the pattern dimensions obtained from the SEM image greatly vary between the center and end of the scan region. The use of such an SEM image not only increases a measurement error, but also frequently causes pseudo-defects in parts where the divergence from the CAD data is great in the inspection of the comparison with the CAD data.
In order to solve the foregoing problem, there has been proposed a method comprising: acquiring an image in a region where there are patterns of the same dimensions and of uniform density on a wafer, previously finding a variation of the dimensions dependent on the position in the scan region, and using the variation to correct the pattern dimensions (e.g., Japanese Patent Laid Open (kokai) No. 2005-277395).
However, the diameter of the electron beam greatly changes with time depending on the size of the scan region and the length of inspection time. Thus, if, for example, a high probe current is used to carry out a long-time inspection or measurement, a device drift (e.g., defocus and astigmatism) occurs which is mainly caused by, for example, a charge-up. This not only changes the beam diameter with time and fluctuates, during an inspection, the variation of the dimensions dependent on the position in the scan region, but also varies the pattern dimensions in an inspection region and precludes a sufficient correction, leading to problems of measurement errors and pseudo-defects.
To correct the above-mentioned change of the beam diameter with time, Japanese Patent Laid Open (kokai) No. 2005-277395 has also proposed a method to create a part in which inspections are conducted twice in the inspection region, and make a correction using dimensions measured in the first and second inspections.
However, in the method proposed in Japanese Patent Laid Open (kokai) No. 2005-277395, the dimensional variation with time which depends on the position in the scan region can not be corrected, and the dimensions still vary with time even during the second inspection, so that it has been impossible to obtain sufficient correction accuracy.
For the defocus and astigmatism, there have been proposed a method to make a real-time correction in an inspection process, and a method to make a correction at regular intervals predetermined in the inspection region. However, the real-time correction method increases the inspection time, and the method of correcting at regular intervals is not capable of making a sufficient correction of suddenly and unexpectedly caused defocus and astigmatism.
Furthermore, when the scan region is large, it is highly likely that patterns greatly different in dimensions are included in the same scan region in an actual pattern on the wafer. In that case, if the measured dimensions including errors in the scan region are corrected using measurement values obtained from the patterns greatly different in dimension, this involves a dimensional bias produced depending on how the edge of the SEM image appears depending on the dimensions of the pattern, and a change in the pattern dimensions on the wafer which is produced depending on the size of a design pattern, leading to a problem of an inhibited increase of correction accuracy.